Placing your images in www/img sounds like a good ideal but it will only work when serving locally using ionic serve.
When building your app, the www/img will get deleted unless you make a gulp task to copy the images from the folder you want to the www/build folder as shown in this post.
Images used in html files should be in src/assets/img(recommended) and not www/assets/img(obselete). Image tags would then look like this :
<img src="https://stackoverflow.com/questions/39952214/assets/img/yourimage.jpg" alt="your image">
In ionic 2, the src/assets folder is meant for images and fonts.
This is what the ionic team says in the guide to modify an existing ionic project :
Move www/img to src/assets/img.
Move any other resources you have in www/ to src/assets/.